Geology topics

A. S. McEwen

Publications and source records attributed to A. S. McEwen.

28 records · Page 2Linked to original sources

USGS1996

Galileo photometry of asteroid 243 Ida

Galileo imaging observations over phase angles 19.5?? to 109.8?? are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at ?? = 0.55 ??m by Hapke parameters ????0 = 0.22, h = 0.020, B0 = 1.5, g = -0.33, and ?? = 18?? with corresponding geometric albedo p = 0.21??0.030.01 and Bond albedo AB = 0.081??0.0170.008. Ida's photometric properties are more similar to those of "average S-asteroids" (P. Helfenstein and J. Veverka 1989, Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-??m absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-??m absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in having a slightly higher value of ????0 (0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5?? to 47.6?? suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21). ?? 1990 Academic Press, Inc.

Icarus↗

USGS1996

Galileo's first images of Jupiter and the Galilean satellites

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiter's Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on lo. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

Science↗

USGS1994

Galileo Photometry of Asteroid 951 Gaspra

Galileo images of Gaspra make it possible for the first time to determine a main-belt asteroid's photometric properties accurately by providing surface-resolved coverage over a wide range of incidence and emission angles and by extending the phase angle coverage to phases not observable from Earth. We combine Earth-based telescopic photometry over phase angles 2° ≤ α ≤ 25° with Galileo whole-disk and disk-resolved data at 33° ≤ α ≤ 51° to derive average global photometric properties in terms of Hapke's photometric model. The microscopic texture and particle phase-function behavior of Gaspra's surface are remarkably like those of other airless rocky bodies such as the Moon. The macroscopic surface roughness parameter, θ̄ = 29°, is slightly larger than that reported for typical lunar materials. The particle single scattering albedo, ώ 0 = 0.36 ± 0.07, is significantly larger than for lunar materials, and the opposition surge amplitude, B 0 = 1.63 ± 0.07, is correspondingly smaller. We determine a visual geometric albedo p v = 0.22 ± 0.06 for Gaspra, in close agreement with p v = 0.22 ± 0.03 estimated from Earth-based observations. Gaspra's phase integral is 0.47, and the bolometric Bond albedo is estimated to be 0.12 ± 0.03. An albedo map derived by correcting Galileo images with our average global photometric function reveals subdued albedo contrasts of ±10% or less over Gaspra's northern hemisphere. Several independent classification algorithms confirm the subtle spectral heterogeneity reported earlier (S. Mottola, M. DiMartino, M. Gonano-Beurer, H. Hoffman, and G. Neukum, 1993, Asteroids, Comets, Meteors, pp. 421-424; M. J. S. Belton et al ., 1992, Science 257, 1647-1652). Whole-disk colors (0.41 ≤ λ ≤ 0.99 μm) vary systematically with longitude by about ±5%, but color differences as large as 30% occur locally. Colors vary continuously between end-member materials whose areal distribution correlates with regional topography. Infrared: violet (0.99:0.41-μm) color ratios on Gaspra are strongly correlated with local elevation, being largest at lower elevations and smaller at higher elevations. No correlation was detected between elevation and the green:violet (0.56:0.41-μm) color ratio. Bright materials with a strong 1-μm absorption occur primarily in association with craters along ridges, while darker materials with 30% weaker 1-μm signatures occur downslope. The variations of color and albedo cannot be easily explained by grain-size effects alone or by differences in photometric geometry. The trends observed are consistent with those revealed by laboratory studies of the effects of comminution, glass formation, and segregation of metal from silicate components in chondritic meteorites and also in some silicate mixtures. The relative importance of these various processes on Gaspra remains to be determined.

Icarus↗

USGS1994

The Clementine mission to the Moon: Scientific overview

In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.

Science↗

USGS1992

Triton's surface-atmosphere energy balance

We explore the energetics of Triton's surface-atmosphere system using a model that includes the turbulent transfer of sensible heat as well as insolation, reradiation, and latent heat transport. The model relies on a 1° by 1° resolution hemispheric bolometric albedo map of Triton for determining the atmospheric temperature, the N 2 frost emissivity, and the temperatures of unfrosted portions of the surface consistent with a frost temperature of ≅38 K. For a physically plausible range of heat transfer coefficients, we find that the atmospheric temperature roughly 1 km above the surface is approximately 1 to 3 K hotter than the surface. Atmospheric temperatures of 48 K suggested by early analysis of radio occultation data cannot be obtained for plausible values of the heat transfer coefficients. Our calculations indicate that Triton's N 2 frosts must have an emissivity well below unity in order to have a temperature of ≅38 K, consistent with previous results. We also find that convection over small hot spots does not significantly cool them off, so they may be able to act as continous sources of buoyancy for convective plumes, but have not explored whether the convection is vigorous enough to entrain particulate matter thereby forming a dust devil. Our elevated atmospheric temperatures make geyser driven plumes with initial upward velocities ≤10 m s −1 stagnate in the lower atmosphere. These “wimpy” plumes provide a possible explanation for Triton's “wind streaks.”

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USGS1992

The canyon system on Mars

Individual Martian equatorial troughs are described, and their stratigraphy, geomorphology and structure are discussed. Possible origins and the overall sequence of events are addressed. Wall rock, interior layered deposits, irregular floor deposits, fractured floor material, and surficial deposits are examined. Chasma walls, wall stability, pits and pit chains, tributary canyons, and the transition from troughs to channels are also discussed.

Book chapter↗

USGS1991

Photometric functions for photoclinometry and other applications

Least-squared fits to the brightness profiles across a disk or “limb darkening” described by Hapke's photometric function are found for the simpler Minnaert and lunar-Lambert functions. The simpler functions are needed to reduce the number of unknown parameters in photoclinometry, especially to distinguish the brightness variations of the surface materials from that due to the resolved topography. The limb darkening varies with the Hapke parameters for macroscopic roughness ( θ ) "> (θ) , the single-scattering albedo ( w ), and the asymmetry factor of the particle phase function ( g ). Both of the simpler functions generally provide good matches to the limb darkening described by Hapke's function, but the lunar-Lambert function is superior when viewing angles are high and when ( θ ) "> (θ) is less than 30°. Although a nonunique solution for the Minnaert function at high phase angles has been described for smooth surfaces, the discrepancy decreases with increasing ( θ ) "> (θ) and virtually disappears when ( θ ) "> (θ) reaches 30° to 40°. The variation in limb darkening with w and g , pronounced for smooth surfaces, is reduced or eliminated when the Hapke parameters are in the range typical of most planetary surfaces; this result simplifies the problem of photoclinometry across terrains with variable surface materials. The Minnaert or lunar-Lambert fits to published Hapke models will give photoclinometric solutions that are very similar (>1° slope discrepancy) to the Hapke-function solutions for nearly all of the bodies and terrains thus far modeled by Hapke's function.

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USGS1989

Mobility of large rock avalanches: evidence from Valles Marineris, Mars

Measurements of H/L (height of drop/length of runout) vs. volume for landslides in Valles Marineris on Mars show a trend of decreasing H/L with increasing volume. This trend, which is linear on a log-log plot, is parallel to but lies above the trend for terrestrial dry rock avalanches. This result and estimates of 10 4 to 10 5 Pa yield strength suggest that the landslides were not water saturated, as suggested by previous workers. The offset between the H/L vs. volume trends shows that a typical Martian avalanche must be nearly two orders of magnitude more voluminous than a typical terrestrial avalanche in order to achieve the same mobility. This offset might be explained by the effects of gravity on flows with high yield strengths. These results should prove useful to future efforts to resolve the controversy over the mechan-ics of long-runout avalanches.

Geology↗

USGS1988

Global color and albedo variations on Io

Three multispectral mosaics of Io have been produced from Voyager imaging data: a global mosaic from each of the Voyager 1 and Voyager 2 data sets and a high-resolution mosaic of the region surrounding the volcano Ra Patera. The mosaics are maps of normal albedo and color in accurate geometric map formats. Io's photometric behavior, mapped with a two-image technique, is spatially variable, especially in the bright white areas. The disk-integrated color and albedo of the satellite have been remarkably constant over recent decades, despite the volcanic activity and the many differences between Voyager 1 and 2 images (acquired just 4 months apart). This constancy is most likely due to the consistent occurrence of large Pele-type plumes with relatively dark, red deposits in the region from long 240 to 360°. A transient brightening southeast of Pele during the Voyager 1 encounter was probably due to real changes in surface and/or atmospheric materials, rather than to photometric behavior. The intrinsic spectral variability of Io, as seen in a series of two-dimensional histograms of the multispectral mosaics, consists of continuous variation among three major spectral end members. The data were mapped into five spectral units to compare them with laboratory measurements of candidate surface materials and to show the planimetric distributions. Unit 1 is best fit by the spectral reflectance of ordinary elemental sulfur, and it is closely associated with the Peletype plume deposits. Unit 2 is strongly confined to the polar caps above about latitude ±50°, but its composition is unknown. Unit 5 is probably SO 2 with relatively minor contamination; it is concentrated in the equatorial region and near the long-lived Prometheus-type plumes. Units 3 and 4 are gradational between units 1 and 5. In addition to SO 2 and elemental sulfur, other plausible components of the surface are polysulfur oxides, FeCl 2 , Na 2 S, and NaHS.

Icarus↗

USGS1983

Two classes of volcanic plumes on Io

Comparison of Voyager 1 and Voyager 2 images of the south polar region of Io has revealed that a major volcanic eruption occured there during the period between the two spacecraft encounters. An annular deposit ∼1400 km in diameter formed around the Aten Patera caldera (311°W, 48°S), the floor of which changed from orange to red-black. The characteristics of this eruption are remarkably similar to those described earlier for an eruption centered on Surt caldera (338°W, 45°N) that occured during the same period, also at high latitude, but in the north. Both volcanic centers were evidently inactive during the Voyager 1 and 2 encounters but were active sometime between the two. The geometric and colorimetric characteristics, as well as scale of the two annular deposits, are virtually identical; both resemble the surface features formed by the eruption of Pele (255°W, 18°S). These three very large plume eruptions suggest a class of eruption distinct from that of six smaller plumes observed to be continously active by both Voyagers 1 and 2. The smaller plumes, of which Prometheus is the type example, are longer-lived, deposit bright, whitish material, erupt at velocities of ∼0.5 km sec −1 , and are concentrated at low latitudes in an equatorial belt around the satellite. The very large Pele-type plumes, on the other hand, are relatively short-lived, deposit darker red materials, erupt at ∼1.0 km sec −1 , and (rather than restricted to a latitudinal band) are restricted in longitude from 240° to 360°W. Both direct thermal infrared temperature measurements and the implied color temperatures for quenched liquid sulfur suggest that hot spot temperatures of ∼650°K are associated with the large plumes and temperatures <400°K with the small plumes. The typical eruption duration of the small plumes is at least several years; that of the large plumes appears to be of the order of days to weeks. The two classes therefore differ by more than two orders of magnitude in duration of eruption. Based on uv, visible, and infrared spectra, the small plumes seem to contain and deposit SO 2 in their annuli whereas the large plumes apparently do not. Two other plumes that occur at either end of the linear feature Loki may be intermediate or hybrid between the two classes, exhibiting attributes of both. Additionally, Loki occurs in the area of overlap in the regional distributions of the two plume classes. Two distinct volcanic systems involving different volatiles may be responsible for the two classes. We propose that the discrete temperatures associated with the two classes are a direct reflection of sulfur's peculiar variation in viscosity with temperature. Over two temperature ranges (∼400 to 430°K and >650°K), sulfur is a low-viscosity fluid (orange and black, respectively); at other temperatures it is either solid or has a high viscosity. As a result, there will be two zones in Io's crust in which liquid sulfur will flow freely: a shallow zone of orange sulfur and a deeper zone of black sulfur. A low-temperature system driven by SO 2 heated to 400 to 400°K by the orange sulfur zone seems the best model for the small plumes; a system driven by sulfur heated to >650°K by hot or even molten silicates in the black sulfur zone seems the best explanation for the large plume class. The large Pele-type plumes are apparently concentrated in a region of the satellite in which a thinner sulfur-rich crust overlies the tidally heated silicate lithosphere, so the black sulfur zone may be fairly shallow in this region. The Prometheus-type plumes are possibly confined to the equatorial belt by some process that concentrates SO 2 fluid in the equatorial crust.

Icarus↗